Electronics > Beginners
Oscillator works when I touch it?
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booyeah:
Hi.

Maybe this is obvious for others but I'm a bit stumped.
I have a circuit consisting of a charge pump / voltage doubler which is connected to three LC tank circuits (two have trimmer caps) in series,
It is then modulated via a dual transistor pair and subsequently capacitively coupled to a high voltage DC bias circuit which drives an avalanche photodiode transimpedance amplifier circut.

Whilst probing the circuit I found that touching the coupling capacitor and/or ground, that the circuit would generate the correct signal and the device would fully function, (don't have schematics / part specs etc.)

I figured it must be a grounding issue and tried adding all manner of capacitors  but none of them helped, seemingly the opposite, adjusting the trimmers also doesn't help.

Any thoughts appreciated :)

alsetalokin4017:
Clean, inspect under microscope, reflow junctions in suspect circuit segment...     :horse:
thedoc298:
Masure your meter and add the same value to your circuit.
booyeah:
I used that old photo just to save me fiddling about making a new one, but I have already tinned all the solder joints, and cleaned the nasty flux residue.

The circuit doesn't start working when probed with an oscilloscope probe.
It's only when I touch the NPN transistor collector with a metal tweezers that it functions.

I don't have a good enough grasp on how these transistor oscillators work to know is it my body capacitance or resistance that's affecting the LC circuit or is it that I'm injecting mains hum by touching it?

Adding small  values of capacitance to ground just seems to attenuate the collector voltage to nothing.

The first image is the standard non - functional behavior,
Channel 2(pink) is a square wave coupled to the transistor base
Channel 4(green) is the resulting collector voltage that gets coupled to a subsequent circut
Channel 1(yellow) is the non-functional result of the subsequent circuits

The second image is when I touch the collector with a metal  tweezers
The ac amplitude of the collector voltage is much reduced and the device initiates a change in frequency from ~14.5MHz to ~500kHz
The device also functions correctly
ArthurDent:
You may have accidentally created the equivalence of a 'THEREMIN' where changing some value changes the circuit. Here's a video showing how one works and what happens sounds something like what you describe.

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